1. Field of the Invention
The invention relates to a method and an apparatus for the determination of characteristic layer parameters during the coating process by means of spectral-optical measurements.
2. Discussion of the Background
Measurements of layer thickness belong to the most important tools during quality control in the semiconductor production. Recently these measurements are not only carried out for quality assurance after the process steps, but also through real-time measurements during the coating processes. In this respect different spectroscopic-optical real-time processes are known.
Within these processes light is suitably irradiated on to the layer stack to be investigated and then measured either in reflection or in transmission. Typical measurement methods using perpendicular incidence of light are: transmission-spectroscopy and reflectance-anisotropy-spectroscopy (RAS). Typical measurement methods using oblique incidence of light are: ellipsometry and polarisation dependent photometry. The here produced change of the light intensity or the light phase caused by the layer structure is measured. This change of the light intensity or of the light phase can be described by physical laws and thus is an unequivocal function of the layer parameters, i.e. the layer thickness and the material used (Born/Wolf, Principles of Optics). By inversion of the arguments the single layer parameter can be determined using this functional dependence. As there is a non-linear dependence between the change in light intensity/light phase and the layer parameters, the mathematical determination is not performed by analytical calculations but by numerical program-algorithms as for example Marquardt-Levenberg, Simplex after Nelder & Meat (Numerical Recipes in C).
It is furthermore known, that the properties of all known layer materials (refraction index n, absorption index k) change with the wavelength of the light as well as in dependence of temperature. Therefore for high temperature processes, as for example a coating procedure, it is necessary to know exactly the temperature of the substrate to be coated (subsequently called sample) in order to analyse the spectroscopic-optical real-time measurement in a correct way. One possibility is to determine numerically the temperature because of the mathematical dependence from the changes in light intensity/light phase as well as an additional fit parameter. By doing that the sample temperature can be determined only with a precision of ±10 K. Unfavourable is, that also the precision of the characteristic layer parameter (for example the layer thickness) is strongly limited because of the mathematical implications.
For a more exact determination of the sample temperature and thereby of the layer parameters the use of pyrometers is known, because a direct temperature measurement, for example by using calibrated platinum resistance or other contact thermometer, as thermocouples, is not possible during a coating process.
As the thermal radiation that is measured in the pyrometer interferes at the growing layer, the measured radiation intensity depends not only on the temperature but also on the layer thickness. This leads to the fact, that the pyrometer signal oscillates during the coating process because of the changing layer thickness, even if the true temperature stays constant. However, this pyrometer signal can be corrected with respect to the emissivity of the sample, as known from DE 44 19 476 C2. Therefore suitable monochromatic light is irradiated on to a sample and from the reflected part the reflectance is determined. From this the emissivity εP of the sample is calculated according to known physical laws (it is assumed that the substrate to be coated is not transparent for the pyrometer wavelength). From the gained measuring data the absolute sample temperature can be determined with an accuracy of temperature measurement of ±1 K and better.
Only by this exact determination of the absolute temperature of the sample the selected layer parameters can be determined with an high accuracy by analysis of the reflected light.
However, using the method proposed in DE 44 19 476 C2 it is unfavourable that the superposed signals from thermal radiation and reflected radiation have to be separated in a complex manner before they can be measured and analysed. This detection of thermal radiation and reflected radiation that has necessarily to be performed separately is carried out in DE 44 19 476 C2 by a complex arrangement by means of twofold phase sensitive frequency modulation using so called chopper and lock-in amplifier. Because of this twofold modulation typical it is not possible to measure exactly industrial applications, which depend on a high resolution in time (as for example fast rotating samples in multi wafer reactors) because it is impossible to distinguish between sample carrier and sample due to the inevitably reduced resolution in time of the detection system. For this reason in WO 02/26435 A1 an arrangement was proposed, where a distinction between sample temperature and sample carrier temperature becomes possible by using a combination of several pyrometers. However, the simultaneous use of several pyrometers is a very complex and cost intensive solution as well.
Furthermore for all solutions proposed for the measurement of the sample temperature, it is disadvantageous that the emissivity of the sample, which influences the pyrometer signal, cannot be measured exactly under industrial conditions (rotating wobbling samples). This might possibly lead to an inaccurate measurement of the sample temperature along with an inaccurate determination of the characteristic layer parameters.